This
electrometer/galvanometer scale, obtained from the CzechRepublic, dates from the
first half of the twentieth century. The manufacturer is unknown. It is
relatively small, ca. 8 “ long and 8” high, and is divided into 100
divisions on either side of zero (located in the center of the scale).

This
type of scale was used to monitor the movement of a beam of light that was
reflected off the mirror of a quadrant electrometer or reflecting
galvanometer. When radiation was being measured, the goal was to quantify
the current from an ion chamber connected to the electrometer.This was usually accomplished by measuring the time it took the
reflected beam to move across a specific number of divisions on the scale.

Until
1860 or so, the procedure was to measure the deflection of the
galvanometer (quadrant electrometers had yet to be invented) by viewing
the reflected image of the scale via a telescope. It worked, but it was an
inexact and cumbersome process. The first to employ a reflected beam of
light was Lord Kelvin (William Thompson).The story is that he conceived the idea while watching a beam of
light that was reflected off the monocle that hung by a ribbon from his
neck.

Reference:

Sylvanus
P. Thompson, The Kelvin Lecture: The Life and Work of Lord Kelvin,
The 1908 Annual Report of The Smithsonian Institution, published 1909,
page 745.